When coating automobile body parts by electrodeposition and otherwise coating metal surfaces, the practice is to prime the surfaces by chemical conversion. Chemical conversion covers a metal surface by an inert conversion coating to improve the adhesion and corrosion resistance of the coating formed over it.
Further, from the viewpoints of reducing the weight of automobiles and ensuring safety, high strength thin gauge steel sheet is used. For chassis parts etc., hot rolled steel sheet less expensive than cold rolled steel sheet is being used.
Hot rolled steel sheet is produced through hot rolling and pickling steps. In the pickling step, the oxide scale on the steel sheet surface is removed by hydrochloric acid pickling.
The following proposals have been made in the past regarding high strength hot rolled steel sheet improved in chemical convertibility and a method of production of the same. For example, Japanese Patent Publication (A) No. 11-50187 discloses high strength hot rolled steel sheet where the ratio of Si concentration at the surface and inside of the steel sheet is made 1.3 or less to eliminate the problems of the deterioration of the chemical convertibility and the resultant deterioration of the corrosion resistance after coating. As means for obtaining such a ratio of Si concentration, grinding the hot rolled steel sheet after pickling etc. to reduce the Si oxides present on the surface is shown.
Further, Japanese Patent Publication (A) No. 10-1748 discloses high strength hot rolled steel sheet where the ratio of the Vicker's hardness of the surface and inside of the steel sheet is made 0.95 or less so as to improve the chemical convertibility and workability. This covers Ti-containing steel. The deposits on the steel sheet surface are made carbides instead of the Ti oxide TiO2 causing deterioration of the chemical convertibility and thereby obtain the above ratio of hardness. It is considered that TiO2 forms conforming fine deposits and raises the hardness of the steel sheet, while TiC is nonconforming and reduces the hardness of the steel sheet. As the means for this, the hot rolling conditions are shown.
Japanese Patent Publication (A) No. 11-50187
Japanese Patent Publication (A) No. 10-1748